In one illustrative application of the invention, in a helicopter personnel rescue hoist, a boom mounted on the upper end portion of a rotatable post or stanchion in a helicopter carries a reel of cable, the free end of which may bear a suspension hook which can be lowered from or elevated back to the free end of the boom. The hook is engageable with a personnel sling. The boom stanchion is swivelled on a vertical axis between the ceiling and floor of a helicopter compartment, and is capable of being rotated, by motor drive, through an angle of substantially 90.degree. between an outwardly projecting rescue position, and an inward unloading position inside the helicopter.
Owing to presence of stanchions or other obstructions, it can sometimes occur that the swinging boom will laterally engage and hang up thereagainst, and it is then desirable to instantly interrupt the drive of the boom by the motor. Steps can then be taken to drive the motor powering the boom in the reverse direction, so that the boom thereupon backs off. It is the primary purpose of the invention to provide a system which will accomplish this operation.